Thinking process

Not very typical backtracking

Mistakes

the number of open should be larger or equals to the number of close tags

public class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> result = new ArrayList<>();

        dfsHelper(result, "", n, n);

        return result;
    }

    private void dfsHelper(List<String> result, String str, int open, int close) {
        if (open > close) return;

        if (open > 0) {
            dfsHelper(result, str + "(", open - 1, close);
        }

        if (close > 0) {
            dfsHelper(result, str + ")", open, close - 1);
        }

        if (open == 0 && close == 0) {
            result.add(str);
            return ;
        }
    }
}